Some semiconductor devices need to adjust the voltage which is supplied from external sources. A solid state imaging device is taken as an example to explain such conventional techniques.
FIG. 5A shows the structure of the conventional solid state imaging device. Photo diodes 1 arranged two-dimensionally are connected to vertical charge coupled devices (CCDs) 2 in every line. The vertical CCDs 2 are connected to a horizontal CCD 3. The horizontal CCD 3 is connected to an output amplifier 4 for charge-detection. The signal charge generated by being converted photoelectrically in the photo diodes 1 is transmitted to the vertical CCDs 2 and then to the horizontal CCD 3, and outputted after being converted to a voltage by the output amplifier 4. A solid state imaging device having this structure contains two portions where the driver voltage should be adjusted. The examples are a blooming control voltage and a reset voltage of a charge-detector of the output amplifier 4.
The following is an explanation about the blooming voltage. When a strong light beam enters, signal electrons generated at the photo diodes overflow and flow into the adjacent photo diodes or the vertical CCDs. This phenomenon is called blooming. A vertical overflow drain or a horizontal overflow drain is used in order to control this blooming. The vertical overflow drain has a structure to discharge the signal electrons to the substrate before the electrons flow into the adjacent photo diodes or the vertical CCDs, while the horizontal overflow drain has a structure to discharge the signal electrons to the drain before they flow into the adjacent photo diodes or the vertical CCDs by providing an exclusive drain and a control electrode.
The vertical overflow drain is briefly explained below. FIG. 5B is a cross-sectional view of a photo diode and an adjacent vertical CCD of the vertical overflow drain, which is taken in line of A-A' of FIG. 5A. FIG. 5C is a graph to show a potential along the line of B-B'-B" of FIG. 5B. Signal electrons 15 are stored at the photo diode 7 in proportion to the amount of incident light. As shown in the potential 12, the signal electrons 15 stored at the photo diode 7 partially flow into the vertical CCD 10 if the potential of the p type area 6 is lower than the barrier potential (9 and 6 of the FIG. 5C) between the photo diode 7 and the vertical CCD 10. As shown in the potential 14, the potential of the p type area 6 is raised and the signal electrons 15 are discharged to the substrate before the electrons 15 are overflowed to the vertical CCD 10 if the voltage 16 applied to the silicon substrate 5 is increased. As mentioned above, the blooming control capability is improved as the voltage 16 to be applied to the substrate 5 is raised, however, the amount of the signal electrons 15 stored at the diode 7 (e.g. saturation signal) is reduced. Therefore, the potential 13 shown in FIG. 5C is the best voltage to obtain the largest saturation signal (storage) while controlling the blooming. In FIG. 5B, 8 and 9 are p type areas and 11 is an electrode.
The conventional semiconductor devices, however, have some problems as follows.
First, the best voltage for every chip varies due to several reasons like the variation of the manufacturing process when the best voltage is applied to the substrate or the control electrode. Thus, it is necessary to adjust the voltage in the external circuit, e.g. the blooming control in the above-mentioned solid state imaging device. In addition to that, the information of the obtained best voltage cannot be maintained inside the semiconductor device though the best voltage can be detected by varying the voltage applied from the outside, according to the conventional method of inspecting a semiconductor device.